1. Field of the Invention
This invention relates to an electrostatic photographic copying apparatus, and more specifically, to an electrostatic photographic copying apparatus of such a construction that a one-cycle copying operation is completed while a rotary drum equipped with a photosensitive element rotates through two turns.
2. Technical Considerations and Prior Art
The conventional electrostatic photographic copying apparatus equipped with a rotary drum are constructed such that a one-cycle copying operation is completed while the rotary drum rotates once. In an electrostatic photographic copying apparatus of this type, an image-forming zone for forming an image on the rotary drum and a transfer zone for transferring the image formed on the rotary drum to a transfer paper are disposed along the outer periphery of the rotary drum, and the copying operation begins when the forwarding end point of the area of photosensitive drum where an image is to be formed is located at a predetermined position upstream of the image-forming zone, and completes when the rotary drum has rotated once. It is necessary at this time that the rear end of the image-forming area of the rotary drum passes the transfer zone when the copying operation has ended. For this reason, the peripheral surface of the rotary drum can be utilized as an image-forming area only at a portion corresponding to the length which is equal to the distance between the downstream end of the transfer zone and the above-mentioned standard position upstream of the image-forming zone in the rotating direction of the rotary drum. In other words, the area on the peripheral surface having a length corresponding to the distance between the standard position upstream of the image-forming zone to the downstream end of the transfer zone in the rotating direction of the rotating drum cannot be utilized as an image-forming area. Thus, in such a type of copying machine, a photosensitive element is attached to the rotary drum to cover about the half of the peripheral length, and forms an image-forming area. This makes it necessary to use a rotary drum of larger diameter to obtain the desired maximum copying length (the length of the image-forming area), and constitutes a setback against the provision of a compact copying machine.
In an attempt to remove such defects, consideration may be given to a system wherein an endless photosensitive layer is provided on the entire surface of the rotary drum, and the rotary drum is started from any desired angular position and rotated by an angle corresponding to the desired copying length. According to this method, however, the angular position of the rotary drum at the time of initiating the copying operation and the angular position of the rotary drum at the completion of the copying operation are not constant, but vary at random. Thus, the control mechanism for the start of supply of transfer paper and its cutting, and the exposure of the original which are all performed in synchronization with the rotary drum becomes very complicated. Furthermore, the provision of an endless and jointless photosensitive layer on the rotary drum is generally costly.